The present disclosure relates to displays based upon encapsulated bichromal balls and methods of manufacturing such displays. It finds particular application in conjunction with display technology, and “electric paper” and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications.
Bichromal balls or beads as sometimes referred to in the art, are small spherical balls which have an optical and an electrical anisotropy. These characteristics generally result from each hemisphere surface having a different color and electrical charge.
These spherical particles are imbedded in a solid substrate and a slight space between each ball and the substrate is filled with an oil so that the balls are free to rotate, and migrate in an oil-filled cavity, in a changing electrical field. If one hemisphere is black and the other is white, each pixel can be turned on and off by the electrical field applied to that location. Each pixel can be individually addressed, and a full page image can thus be generated.
Numerous patents describe bichromal balls, their manufacture, incorporation in display systems or substrates, and related uses and applications. Exemplary patents include, but are not limited to: U.S. Pat. Nos. 5,262,098; 5,344,594; 5,604,027 reissued as Re 37,085; 5,708,525; 5,717,514; 5,739,801; 5,754,332; 5,815,306; 5,900,192; 5,976,428; 6,054,071; 5,989,629; 6,235,395; 6,419,982; 6,235,395; 6,419,982; 6,445,490; and 6,703,074; all of which are hereby incorporated by reference. In addition, disclosure is provided by U.S. Pat. Nos. 4,126,854; and 5,825,529; and N. K. Sheridon et al., “The Gyricon—A twisting ball display”, Proc. SID, Boston, Mass., 289, 1977; T. Pham et al., “Electro-optical characteristics of the Gyricon display”, SID ‘02 Digest, 199, 2002; all of which are hereby incorporated by reference.
Gyricon display technology is currently being developed for commercial signage application. The Gyricon display devices are often produced by the “swollen sheet” method. In this method, the Gyricon display sheets are made by mixing or dispersing bare bichromal balls, which are approximately 75 μm to 150 μm in diameter, into an uncured silicone elastomer to yield a viscous paste. The elastomeric gel is then spread into a thin layer by a doctor blade on a supporting substrate and cured to form a solid flexible sheet. The sheet is then soaked in a dielectric oil that causes the elastomer to swell and form an oil-filled cavity around each ball. The balls are free to rotate in each swollen cavity. The swollen sheets are sandwiched between substrates carrying arrays of addressing electrodes to form a display.
Fabrication of the Gyricon display sheet involves many complicated steps. The material into which the balls are disposed or embedded is largely limited to the class of elastomeric materials chosen for their large capacity to swell. This material is costly. To prevent the loss of dielectric oil, which would render the display inoperative, the display needs to be sealed. The produced Gyricon sheet is relatively thick, about 300 μm to about 700 μm, and includes a dispersion of bichromal balls of many layers. The disadvantage of the multi-layered structure is that it degrades the contrast, i.e. reflectance, degrades the resolution, increases the switching voltage, and increases the complexity and cost of the driver electronics of the display. These add to the complexity and cost of the resulting Gyricon display.
Additionally, U.S. Pat. No. 6,445,496, which is incorporated herein by reference in its entirety, describes a method of encapsulating the bichromal balls within an oil-filled capsule. The capsule is formed by chemical means. Encapsulation of the Gyricon beads may eliminate the need for a costly elastomer and the sealing steps.
Furthermore, U.S. Pat. No. 6,492,025, incorporated herein by reference in its entirety, describes an example of microcapsule composition. U.S. Pat. No. 6,488,870, also incorporated herein by reference in its entirety, describes examples of additional encapsulation processes.
Monolayers of encapsulated bichromal balls can also be produced. The balls are generally randomly dispersed within the resin. However, such randomly dispersed balls are not closely-packed and produce a large amount of open space. This results in decreased optical contrast.
Accordingly, there is a need for a display member of one or more monolayers of closely-packed bichromal balls, and a technique for producing such a display member.